Calendering is an economic and highly efficient means to produce film and sheet from plastics such as plasticized and rigid poly(vinyl chloride), abbreviated herein as “PVC”, and polypropylene compositions. The film and sheet usually have a thickness ranging from about 2 mils (0.05 mm) to about 80 mils (2.0 mm). Calendered PVC film or sheet are readily thermoformed into various shapes that can be used in a wide range of applications including packaging, pool liners, graphic arts, transaction cards, security cards, veneers, wall coverings, book bindings, folders, floor tiles, and products which are printed, decorated, or laminated in a secondary operation. Additional discussion on polypropylene resin compositions used in calendering processes may be found in Japan Application No. Hei 7-197213 and European Patent Application No. 0 744 439 A1.
By contrast, conventional processing of polyesters into film or sheet involves extruding a polyester melt through a manifold of a flat die. Manual or automatic die lip adjustment is used to control thickness across a web of material. Water-cooled chill rolls are used to quench the molten web and impart a smooth surface finish. Although extrusion processes produce film and sheet of excellent quality, extrusion methods do not have the throughput and economic advantages of calendering processes.
PVC compositions are, by far, the largest segment of the calendered film and sheet business. Small amounts of other thermoplastic polymers such as, for example, thermoplastic rubbers, certain polyurethanes, talc-filled polypropylene, acrylonitrile/buta-diene/styrene terpolymers (ABS resins), and chlorinated polyethylene, are sometimes processed by calendering methods. By contrast, polyester polymers such as, for example, poly(ethylene terephthalate), abbreviated herein as “PET”, or poly(1,4-butylene terephthalate), abbreviated herein as “PBT”, are difficult to calender successfully. For example, PET polymers with inherent viscosity values of about 0.6 deciliters/gram (abbreviated herein as “dL/g”), typically have insufficient melt strength to perform properly on the calendering rolls. Melt strength is defined as the ability of a polymer to support its weight in the molten state. In calendering, melt strength is related to the ability to remove the film from the roll process without deformation. For example, when calendered, a polymer with low melt strength will quickly sag and hit the floor; whereas, a polymer with high melt strength will maintain its shape for a much longer amount of time and can be further processed. Melt strength is thus important to minimize the amount of “drawdown” and gravity-induced sagging the polymer experiences during the calendering process. Drawdown is defined in calendering as the amount of thickness reduction between the calendering rolls and the take-up system and is expressed as the ratio of the nominal thickness or width dimension as the film exits the calendering rolls with the same dimension at the take up roles. Also, PET and other polyester polymers are prone to crystallize at typical processing temperatures of 160° C. to 180° C., resulting a non-homogeneous mass which also causes high forces on the calender bearings. Increasing processing temperatures will reduce melt viscosity and improve processability. Higher temperatures, however, can cause degradation of the polyester such as, for example, by thermal degradation, hydrolysis of polymer by exposure to atmospheric moisture, and the formation of color bodies. Typical PET polymers also have a tendency to stick to the calendering rolls at higher processing temperatures. The calendering of various polyester compositions and several approaches to these problems has been described, for example, in U.S. Pat. Nos. 5,998,005; 6,068,910; 6,551,688; U.S. patent application Ser. No. 10/086,905; Japan Patent Application Nos. 8-283547; 7-278418; 2000-243055; 10-363-908; 2000-310710; 2001-331315; 11-158358; and World Patent Application No. 02/28967. Although some these difficulties can be avoided by the careful selection of polymer properties, additives, and processing conditions, calendering of polyesters at high rates of production is difficult.
The rate of production in a calendering process, usually referred to as line speed, is determined by several factors. Equipment design and capability, for example, will have a large influence on how fast and efficient a calendering process will run. Absent any equipment limitations, however, the line speed and efficiency of a calendering process is highly dependent on the material being run.
The higher the line speed, the greater the chances that melt fracture will result. Melt fracture gives a rough, frosty or hazy appearance to the material and is the result of the material not being able to respond to the shear applied during the process. Melt fracture occurs whenever the wall shear stress on the calendering roll exceeds a certain value (typically 0.1 to 0.2 MPa) and the onset of melt fracture is often the rate determining step in a calendering process. Shear stress is controlled by the volume throughput or line speed (which dictates the shear rate) and the viscosity of the polymer melt. By reducing either the line speed or the viscosity at high shear rates, the wall shear stress is reduced and the chance for melt fracture is lowered. Reducing shear stress, therefore, will reduce the chances of melt fracture as the line speed of a calendering process is increased. Reducing shear stress and melt fracture in polyesters has been addressed in extrusion processes. For example, U.S. Pat. No. 6,632,390 describes a process for producing a profile extrusion in which the processability of the polyester composition is improved by the addition of a branching agent, which provides increased melt strength and increased high shear thinning. The polyester composition has an inherent viscosity of at least 0.65 dL/g. Polyester polymers, however, often show a relatively flat shear-thinning response (i.e., there is little change in the melt viscosity of the polymer between low and high shear rates) in calendering processes in comparison to polymers typically processed by calendering such as, for example, PVC or polypropylene. Thus, if a polyester with a higher melt viscosity is used to obtain sufficient melt strength, insufficient shear thinning often causes unacceptably high forces on the calender bearings. Increasing the processing temperature can reduce the occurrence of melt fracture in calendering but, as noted above, also can result in polymer degradation and an unsatisfactory polymer melt strength. Thus, the difficulties presented by shear response and melt-strength frequently prevent polyester polymers from being calendered at high line speeds and/or lower processing temperatures where the highest product quality and lowest production cost may be obtained. To address these problems, a polyester that is capable of being calendered at high line speeds and/or at lower processing temperatures is needed.